Message-ID: <CALyZvKyq1MOsdM9j_VoXjG8rR8tQ747xY8m-SAy8haY0o8406A@mail.gmail.com>
Date:   Wed, 22 Feb 2017 20:26:30 +0000
From:   Jason Vas Dias <jason.vas.dias@...il.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     kernel-janitors@...r.kernel.org,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Prarit Bhargava <prarit@...hat.com>, x86@...nel.org
Subject: Re: [PATCH] arch/x86/kernel/tsc.c : set X86_FEATURE_ART for TSC on
 CPUs like i7-4910MQ : bug #194609

OK, last post on this issue today -
can anyone explain why, with standard 4.10.0 kernel & no new
'notsc_adjust' option, and the same maths being used, these two runs
should display
such a wide disparity between clock_gettime(CLOCK_MONOTONIC_RAW,&ts)
values ? :

$ J/pub/ttsc/ttsc1
max_extended_leaf: 80000008
has tsc: 1 constant: 1
Invariant TSC is enabled: Actual TSC freq: 2.893299GHz - TSC adjust: 1.
ts2 - ts1: 162 ts3 - ts2: 110 ns1: 0.000000641 ns2: 0.000002850
ts3 - ts2: 175 ns1: 0.000000659
ts3 - ts2: 18 ns1: 0.000000643
ts3 - ts2: 18 ns1: 0.000000618
ts3 - ts2: 17 ns1: 0.000000620
ts3 - ts2: 17 ns1: 0.000000616
ts3 - ts2: 18 ns1: 0.000000641
ts3 - ts2: 18 ns1: 0.000000709
ts3 - ts2: 20 ns1: 0.000000763
ts3 - ts2: 20 ns1: 0.000000735
ts3 - ts2: 20 ns1: 0.000000761
t1 - t0: 78200 - ns2: 0.000080824
$ J/pub/ttsc/ttsc1
max_extended_leaf: 80000008
has tsc: 1 constant: 1
Invariant TSC is enabled: Actual TSC freq: 2.893299GHz - TSC adjust: 1.
ts2 - ts1: 217 ts3 - ts2: 221 ns1: 0.000001294 ns2: 0.000005375
ts3 - ts2: 210 ns1: 0.000001418
ts3 - ts2: 23 ns1: 0.000001399
ts3 - ts2: 22 ns1: 0.000001445
ts3 - ts2: 25 ns1: 0.000001321
ts3 - ts2: 20 ns1: 0.000001428
ts3 - ts2: 25 ns1: 0.000001367
ts3 - ts2: 23 ns1: 0.000001425
ts3 - ts2: 23 ns1: 0.000001357
ts3 - ts2: 22 ns1: 0.000001487
ts3 - ts2: 25 ns1: 0.000001377
t1 - t0: 145753 - ns2: 0.000150781

(complete source of test program ttsc1 attached in ttsc.tar
 $ tar -xpf ttsc.tar
 $ cd ttsc
 $ make
).

On 22/02/2017, Jason Vas Dias <jason.vas.dias@...il.com> wrote:
> I actually tried adding a 'notsc_adjust' kernel option to disable any
> setting or
> access to the TSC_ADJUST MSR, but then I see the problems  - a big
> disparity
> in values depending on which CPU the thread is scheduled -  and no
> improvement in clock_gettime() latency.  So I don't think the new
> TSC_ADJUST
> code in ts_sync.c itself is the issue - but something added @ 460ns
> onto every clock_gettime() call when moving from v4.8.0 -> v4.10.0 .
> As I don't think fixing the clock_gettime() latency issue is my problem or
> even
> possible with current clock architecture approach, it is a non-issue.
>
> But please, can anyone tell me if are there any plans to move the time
> infrastructure  out of the kernel and into glibc along the lines
> outlined
> in previous mail - if not, I am going to concentrate on this more radical
> overhaul approach for my own systems .
>
> At least, I think mapping the clocksource information structure itself in
> some
> kind of sharable page makes sense . Processes could map that page
> copy-on-write
> so they could start off with all the timing parameters preloaded,  then
> keep
> their copy updated using the rdtscp instruction , or msync() (read-only)
> with the kernel's single copy to get the latest time any process has
> requested.
> All real-time parameters & adjustments could be stored in that page ,
> & eventually a single copy of the tzdata could be used by both kernel
> & user-space.
> That is what I am working towards. Any plans to make linux real-time tsc
> clock user-friendly ?
>
>
>
> On 22/02/2017, Jason Vas Dias <jason.vas.dias@...il.com> wrote:
>> Yes, my CPU is still getting a fault every time the TSC_ADJUST MSR is
>> read or written . It is probably because it genuinuely does not
>> support any cpuid > 13 ,
>> or the modern TSC_ADJUST interface . This is probably why my
>> clock_gettime()
>> latencies are so bad. Now I have to develop a patch to disable all access
>> to
>> TSC_ADJUST MSR if boot_cpu_data.cpuid_level <= 13 .
>> I really have an unlucky CPU :-) .
>>
>> But really, I think this issue goes deeper into the fundamental limits of
>> time measurement on Linux : it is never going to be possible to measure
>> minimum times with clock_gettime() comparable with those returned by
>> rdtscp instruction - the time taken to enter the kernel through the VDSO,
>> queue an access to vsyscall_gtod_data via a workqueue, access it & do
>> computations & copy value to user-space is NEVER going to be up to the
>> job of measuring small real-time durations of the order of 10-20 TSC
>> ticks
>> .
>>
>> I think the best way to solve this problem going forward would be to
>> store
>> the entire vsyscall_gtod_data  data structure representing the current
>> clocksource
>> in a shared page which is memory-mappable (read-only) by user-space .
>> I think sser-space programs should be able to do something like :
>>     int fd =
>> open("/sys/devices/system/clocksource/clocksource0/gtod.page",O_RDONLY);
>>     size_t psz = getpagesize();
>>     void *gtod = mmap( 0, psz, PROT_READ, MAP_PRIVATE, fd, 0 );
>>     msync(gtod,psz,MS_SYNC);
>>
>> Then they could all read the real-time clock values as they are updated
>> in real-time by the kernel, and know exactly how to interpret them .
>>
>> I also think that all mktime() / gmtime() / localtime() timezone handling
>> functionality should be
>> moved to user-space, and that the kernel should actually load and link in
>> some
>> /lib/libtzdata.so
>> library, provided by glibc / libc implementations, that is exactly the
>> same library
>> used by glibc() code to parse tzdata ; tzdata should be loaded at boot
>> time
>> by the kernel from the same places glibc loads it, and both the kernel
>> and
>> glibc should use identical mktime(), gmtime(), etc. functions to access
>> it,
>> and
>> glibc using code would not need to enter the kernel at all for any
>> time-handling
>> code. This tzdata-library code be automatically loaded into process
>> images
>> the
>> same way the vdso region is , and the whole system could access only one
>> copy of it and the 'gtod.page' in memory.
>>
>> That's just my two-cents worth, and how I'd like to eventually get
>> things working
>> on my system.
>>
>> All the best, Regards,
>> Jason
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>>
>> On 22/02/2017, Jason Vas Dias <jason.vas.dias@...il.com> wrote:
>>> On 22/02/2017, Jason Vas Dias <jason.vas.dias@...il.com> wrote:
>>>> RE:
>>>>>> 4.10 has  new code which utilizes the TSC_ADJUST MSR.
>>>>
>>>> I just built an unpatched linux v4.10 with tglx's TSC improvements -
>>>> much else improved in this kernel (like iwlwifi) - thanks!
>>>>
>>>> I have attached an updated version of the test program which
>>>> doesn't print the bogus "Nominal TSC Frequency" (the previous
>>>> version printed it, but equally ignored it).
>>>>
>>>> The clock_gettime(CLOCK_MONOTONIC_RAW,&ts) latency has improved by
>>>> a factor of 2 - it used to be @140ns and is now @ 70ns  ! Wow!  :
>>>>
>>>> $ uname -r
>>>> 4.10.0
>>>> $ ./ttsc1
>>>> max_extended_leaf: 80000008
>>>> has tsc: 1 constant: 1
>>>> Invariant TSC is enabled: Actual TSC freq: 2.893299GHz.
>>>> ts2 - ts1: 144 ts3 - ts2: 96 ns1: 0.000000588 ns2: 0.000002599
>>>> ts3 - ts2: 178 ns1: 0.000000592
>>>> ts3 - ts2: 14 ns1: 0.000000577
>>>> ts3 - ts2: 14 ns1: 0.000000651
>>>> ts3 - ts2: 17 ns1: 0.000000625
>>>> ts3 - ts2: 17 ns1: 0.000000677
>>>> ts3 - ts2: 17 ns1: 0.000000626
>>>> ts3 - ts2: 17 ns1: 0.000000627
>>>> ts3 - ts2: 17 ns1: 0.000000627
>>>> ts3 - ts2: 18 ns1: 0.000000655
>>>> ts3 - ts2: 17 ns1: 0.000000631
>>>> t1 - t0: 89067 - ns2: 0.000091411
>>>>
>>>
>>>
>>> Oops, going blind in my old age. These latencies are actually 3 times
>>> greater than under 4.8 !!
>>>
>>> Under 4.8, the program printed latencies of @ 140ns for clock_gettime,
>>> as
>>> shown
>>> in bug 194609 as the 'ns1' (timespec_b - timespec_a) value::
>>>
>>> ts3 - ts2: 24 ns1: 0.000000162
>>> ts3 - ts2: 17 ns1: 0.000000143
>>> ts3 - ts2: 17 ns1: 0.000000146
>>> ts3 - ts2: 17 ns1: 0.000000149
>>> ts3 - ts2: 17 ns1: 0.000000141
>>> ts3 - ts2: 16 ns1: 0.000000142
>>>
>>> now the clock_gettime(CLOCK_MONOTONIC_RAW,&ts) latency is @
>>> 600ns, @ 4 times more than under 4.8 .
>>> But I'm glad the TSC_ADJUST problems are fixed.
>>>
>>> Will programs reading :
>>>  $ cat /sys/devices/msr/events/tsc
>>>  event=0x00
>>> read a new event for each setting of the TSC_ADJUST MSR or a wrmsr on
>>> the
>>> TSC ?
>>>
>>>> I think this is because under Linux 4.8, the CPU got a fault every
>>>> time it read the TSC_ADJUST MSR.
>>>
>>> maybe it still is!
>>>
>>>
>>>> But user programs wanting to use the TSC  and correlate its value to
>>>> clock_gettime(CLOCK_MONOTONIC_RAW) values accurately like the above
>>>> program still have to  dig the TSC frequency value out of the kernel
>>>> with objdump  - this was really the point of the bug #194609.
>>>>
>>>> I would still like to investigate exporting 'tsc_khz' & 'mult' +
>>>> 'shift' values via sysfs.
>>>>
>>>> Regards,
>>>> Jason.
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On 21/02/2017, Jason Vas Dias <jason.vas.dias@...il.com> wrote:
>>>>> Thank You for enlightening me -
>>>>>
>>>>> I was just having a hard time believing that Intel would ship a chip
>>>>> that features a monotonic, fixed frequency timestamp counter
>>>>> without specifying in either documentation or on-chip or in ACPI what
>>>>> precisely that hard-wired frequency is, but I now know that to
>>>>> be the case for the unfortunate i7-4910MQ - I mean, how can the CPU
>>>>> assert CPUID:80000007[8] ( InvariantTSC ) which it does, which is
>>>>> difficult to reconcile with the statement in the SDM :
>>>>>   17.16.4  Invariant Time-Keeping
>>>>>     The invariant TSC is based on the invariant timekeeping hardware
>>>>>     (called Always Running Timer or ART), that runs at the core
>>>>> crystal
>>>>> clock
>>>>>     frequency. The ratio defined by CPUID leaf 15H expresses the
>>>>> frequency
>>>>>     relationship between the ART hardware and TSC. If
>>>>> CPUID.15H:EBX[31:0]
>>>>> !=
>>>>> 0
>>>>>     and CPUID.80000007H:EDX[InvariantTSC] = 1, the following linearity
>>>>>     relationship holds between TSC and the ART hardware:
>>>>>     TSC_Value = (ART_Value * CPUID.15H:EBX[31:0] )
>>>>>                          / CPUID.15H:EAX[31:0] + K
>>>>>     Where 'K' is an offset that can be adjusted by a privileged
>>>>> agent*2.
>>>>>      When ART hardware is reset, both invariant TSC and K are also
>>>>> reset.
>>>>>
>>>>> So I'm just trying to figure out what CPUID.15H:EBX[31:0]  and
>>>>> CPUID.15H:EAX[31:0]  are for my hardware.  I assumed (incorrectly)
>>>>> that
>>>>> the "Nominal TSC Frequency" formulae in the manul must apply to all
>>>>> CPUs with InvariantTSC .
>>>>>
>>>>> Do I understand correctly , that since I do have InvariantTSC ,  the
>>>>> TSC_Value is in fact calculated according to the above formula, but
>>>>> with
>>>>> a "hidden" ART Value,  & Core Crystal Clock frequency & its ratio to
>>>>> TSC frequency ?
>>>>> It was obvious this nominal TSC Frequency had nothing to do with the
>>>>> actual TSC frequency used by Linux, which is 'tsc_khz' .
>>>>> I guess wishful thinking led me to believe CPUID:15h was actually
>>>>> supported somehow , because I thought InvariantTSC meant it had ART
>>>>> hardware .
>>>>>
>>>>> I do strongly suggest that Linux exports its calibrated TSC Khz
>>>>> somewhere to user
>>>>> space .
>>>>>
>>>>> I think the best long-term solution would be to allow programs to
>>>>> somehow read the TSC without invoking
>>>>> clock_gettime(CLOCK_MONOTONIC_RAW,&ts), &
>>>>> having to enter the kernel, which incurs an overhead of > 120ns on my
>>>>> system
>>>>> .
>>>>>
>>>>>
>>>>> Couldn't linux export its 'tsc_khz' and / or 'clocksource->mult' and
>>>>> 'clocksource->shift' values to /sysfs somehow ?
>>>>>
>>>>> For instance , only  if the 'current_clocksource' is 'tsc', then these
>>>>> values could be exported as:
>>>>> /sys/devices/system/clocksource/clocksource0/shift
>>>>> /sys/devices/system/clocksource/clocksource0/mult
>>>>> /sys/devices/system/clocksource/clocksource0/freq
>>>>>
>>>>> So user-space programs could  know that the value returned by
>>>>>     clock_gettime(CLOCK_MONOTONIC_RAW)
>>>>>   would be
>>>>>     {    .tv_sec =  ( ( rdtsc() * mult ) >> shift ) >> 32,
>>>>>       , .tv_nsec = ( ( rdtsc() * mult ) >> shift ) >> &~0U
>>>>>     }
>>>>>   and that represents ticks of period (1.0 / ( freq * 1000 )) S.
>>>>>
>>>>> That would save user-space programs from having to know 'tsc_khz' by
>>>>> parsing the 'Refined TSC' frequency from log files or by examining the
>>>>> running kernel with objdump to obtain this value & figure out 'mult' &
>>>>> 'shift' themselves.
>>>>>
>>>>> And why not a
>>>>>   /sys/devices/system/clocksource/clocksource0/value
>>>>> file that actually prints this ( ( rdtsc() * mult ) >> shift )
>>>>> expression as a long integer?
>>>>> And perhaps a
>>>>>   /sys/devices/pnp0/XX\:YY/rtc/rtc0/nanoseconds
>>>>> file that actually prints out the number of real-time nano-seconds
>>>>> since
>>>>> the
>>>>> contents of the existing
>>>>>   /sys/devices/pnp0/XX\:YY/rtc/rtc0/{time,since_epoch}
>>>>> files using the current TSC value?
>>>>> To read the rtc0/{date,time} files is already faster than entering the
>>>>> kernel to call
>>>>> clock_gettime(CLOCK_REALTIME, &ts) & convert to integer for scripts.
>>>>>
>>>>> I will work on developing a patch to this effect if no-one else is.
>>>>>
>>>>> Also, am I right in assuming that the maximum granularity of the
>>>>> real-time
>>>>> clock
>>>>> on my system is 1/64th of a second ? :
>>>>>  $ cat /sys/devices/pnp0/00\:02/rtc/rtc0/max_user_freq
>>>>>  64
>>>>> This is the maximum granularity that can be stored in CMOS , not
>>>>> returned by TSC? Couldn't we have something similar that gave an
>>>>> accurate idea of TSC frequency and the precise formula applied to TSC
>>>>> value to get clock_gettime
>>>>> (CLOCK_MONOTONIC_RAW) value ?
>>>>>
>>>>> Regards,
>>>>> Jason
>>>>>
>>>>>
>>>>> This code does produce good timestamps with a latency of @20ns
>>>>> that correlate well with clock_gettIme(CLOCK_MONOTONIC_RAW,&ts)
>>>>> values, but it depends on a global variable that  is initialized to
>>>>> the 'tsc_khz' value
>>>>> computed by running kernel parsed from objdump /proc/kcore output :
>>>>>
>>>>> static inline __attribute__((always_inline))
>>>>> U64_t
>>>>> IA64_tsc_now()
>>>>> { if(!(    _ia64_invariant_tsc_enabled
>>>>>       ||(( _cpu0id_fd == -1) &&
>>>>> IA64_invariant_tsc_is_enabled(NULL,NULL))
>>>>>       )
>>>>>     )
>>>>>   { fprintf(stderr, __FILE__":%d:(%s): must be called with invariant
>>>>> TSC enabled.\n");
>>>>>     return 0;
>>>>>   }
>>>>>   U32_t tsc_hi, tsc_lo;
>>>>>   register UL_t tsc;
>>>>>   asm volatile
>>>>>   ( "rdtscp\n\t"
>>>>>     "mov %%edx, %0\n\t"
>>>>>     "mov %%eax, %1\n\t"
>>>>>     "mov %%ecx, %2\n\t"
>>>>>   : "=m" (tsc_hi) ,
>>>>>     "=m" (tsc_lo) ,
>>>>>     "=m" (_ia64_tsc_user_cpu) :
>>>>>   : "%eax","%ecx","%edx"
>>>>>   );
>>>>>   tsc=(((UL_t)tsc_hi) << 32)|((UL_t)tsc_lo);
>>>>>   return tsc;
>>>>> }
>>>>>
>>>>> __thread
>>>>> U64_t _ia64_first_tsc = 0xffffffffffffffffUL;
>>>>>
>>>>> static inline __attribute__((always_inline))
>>>>> U64_t IA64_tsc_ticks_since_start()
>>>>> { if(_ia64_first_tsc == 0xffffffffffffffffUL)
>>>>>   { _ia64_first_tsc = IA64_tsc_now();
>>>>>     return 0;
>>>>>   }
>>>>>   return (IA64_tsc_now() - _ia64_first_tsc) ;
>>>>> }
>>>>>
>>>>> static inline __attribute__((always_inline))
>>>>> void
>>>>> ia64_tsc_calc_mult_shift
>>>>> ( register U32_t *mult,
>>>>>   register U32_t *shift
>>>>> )
>>>>> { /* paraphrases Linux clocksource.c's clocks_calc_mult_shift()
>>>>> function:
>>>>>    * calculates second + nanosecond mult + shift in same way linux
>>>>> does.
>>>>>    * we want to be compatible with what linux returns in struct
>>>>> timespec ts after call to
>>>>>    * clock_gettime(CLOCK_MONOTONIC_RAW, &ts).
>>>>>    */
>>>>>   const U32_t scale=1000U;
>>>>>   register U32_t from= IA64_tsc_khz();
>>>>>   register U32_t to  = NSEC_PER_SEC / scale;
>>>>>   register U64_t sec = ( ~0UL / from ) / scale;
>>>>>   sec = (sec > 600) ? 600 : ((sec > 0) ? sec : 1);
>>>>>   register U64_t maxsec = sec * scale;
>>>>>   UL_t tmp;
>>>>>   U32_t sft, sftacc=32;
>>>>>   /*
>>>>>    * Calculate the shift factor which is limiting the conversion
>>>>>    * range:
>>>>>    */
>>>>>   tmp = (maxsec * from) >> 32;
>>>>>   while (tmp)
>>>>>   { tmp >>=1;
>>>>>     sftacc--;
>>>>>   }
>>>>>   /*
>>>>>    * Find the conversion shift/mult pair which has the best
>>>>>    * accuracy and fits the maxsec conversion range:
>>>>>    */
>>>>>   for (sft = 32; sft > 0; sft--)
>>>>>   { tmp = ((UL_t) to) << sft;
>>>>>     tmp += from / 2;
>>>>>     tmp = tmp / from;
>>>>>     if ((tmp >> sftacc) == 0)
>>>>>       break;
>>>>>   }
>>>>>   *mult = tmp;
>>>>>   *shift = sft;
>>>>> }
>>>>>
>>>>> __thread
>>>>> U32_t _ia64_tsc_mult = ~0U, _ia64_tsc_shift=~0U;
>>>>>
>>>>> static inline __attribute__((always_inline))
>>>>> U64_t IA64_s_ns_since_start()
>>>>> { if( ( _ia64_tsc_mult == ~0U ) || ( _ia64_tsc_shift == ~0U ) )
>>>>>     ia64_tsc_calc_mult_shift( &_ia64_tsc_mult, &_ia64_tsc_shift);
>>>>>   register U64_t cycles = IA64_tsc_ticks_since_start();
>>>>>   register U64_t ns = ((cycles
>>>>> *((UL_t)_ia64_tsc_mult))>>_ia64_tsc_shift);
>>>>>   return( (((ns / NSEC_PER_SEC)&0xffffffffUL) << 32) | ((ns %
>>>>> NSEC_PER_SEC)&0x3fffffffUL) );
>>>>>   /* Yes, we are purposefully ignoring durations of more than 4.2
>>>>> billion seconds here! */
>>>>> }
>>>>>
>>>>>
>>>>> I think Linux should export the 'tsc_khz', 'mult' and 'shift' values
>>>>> somehow,
>>>>> then user-space libraries could have more confidence in using 'rdtsc'
>>>>> or 'rdtscp'
>>>>> if Linux's current_clocksource is 'tsc'.
>>>>>
>>>>> Regards,
>>>>> Jason
>>>>>
>>>>>
>>>>>
>>>>> On 20/02/2017, Thomas Gleixner <tglx@...utronix.de> wrote:
>>>>>> On Sun, 19 Feb 2017, Jason Vas Dias wrote:
>>>>>>
>>>>>>> CPUID:15H is available in user-space, returning the integers : ( 7,
>>>>>>> 832, 832 ) in EAX:EBX:ECX , yet boot_cpu_data.cpuid_level is 13 , so
>>>>>>> in detect_art() in tsc.c,
>>>>>>
>>>>>> By some definition of available. You can feed CPUID random leaf
>>>>>> numbers
>>>>>> and
>>>>>> it will return something, usually the value of the last valid CPUID
>>>>>> leaf,
>>>>>> which is 13 on your CPU. A similar CPU model has
>>>>>>
>>>>>> 0x0000000d 0x00: eax=0x00000007 ebx=0x00000340 ecx=0x00000340
>>>>>> edx=0x00000000
>>>>>>
>>>>>> i.e. 7, 832, 832, 0
>>>>>>
>>>>>> Looks familiar, right?
>>>>>>
>>>>>> You can verify that with 'cpuid -1 -r' on your machine.
>>>>>>
>>>>>>> Linux does not think ART is enabled, and does not set the
>>>>>>> synthesized
>>>>>>> CPUID +
>>>>>>> ((3*32)+10) bit, so a program looking at /dev/cpu/0/cpuid would not
>>>>>>> see this bit set .
>>>>>>
>>>>>> Rightfully so. This is a Haswell Core model.
>>>>>>
>>>>>>> if an e1000 NIC card had been installed, PTP would not be available.
>>>>>>
>>>>>> PTP is independent of the ART kernel feature . ART just provides
>>>>>> enhanced
>>>>>> PTP features. You are confusing things here.
>>>>>>
>>>>>> The ART feature as the kernel sees it is a hardware extension which
>>>>>> feeds
>>>>>> the ART clock to peripherals for timestamping and time correlation
>>>>>> purposes. The ratio between ART and TSC is described by CPUID leaf
>>>>>> 0x15
>>>>>> so
>>>>>> the kernel can make use of that correlation, e.g. for enhanced PTP
>>>>>> accuracy.
>>>>>>
>>>>>> It's correct, that the NONSTOP_TSC feature depends on the
>>>>>> availability
>>>>>> of
>>>>>> ART, but that has nothing to do with the feature bit, which solely
>>>>>> describes the ratio between TSC and the ART frequency which is
>>>>>> exposed
>>>>>> to
>>>>>> peripherals. That frequency is not necessarily the real ART
>>>>>> frequency.
>>>>>>
>>>>>>> Also, if the MSR TSC_ADJUST has not yet been written, as it seems to
>>>>>>> be
>>>>>>> nowhere else in Linux,  the code will always think X86_FEATURE_ART
>>>>>>> is
>>>>>>> 0
>>>>>>> because the CPU will always get a fault reading the MSR since it has
>>>>>>> never been written.
>>>>>>
>>>>>> Huch? If an access to the TSC ADJUST MSR faults, then something is
>>>>>> really
>>>>>> wrong. And writing it unconditionally to 0 is not going to happen.
>>>>>> 4.10
>>>>>> has
>>>>>> new code which utilizes the TSC_ADJUST MSR.
>>>>>>
>>>>>>> It would be nice for user-space programs that want to use the TSC
>>>>>>> with
>>>>>>> rdtsc / rdtscp instructions, such as the demo program attached to
>>>>>>> the
>>>>>>> bug report,
>>>>>>> could have confidence that Linux is actually generating the results
>>>>>>> of
>>>>>>> clock_gettime(CLOCK_MONOTONIC_RAW, &timespec)
>>>>>>> in a predictable way from the TSC by looking at the
>>>>>>>  /dev/cpu/0/cpuid[bit(((3*32)+10)] value before enabling user-space
>>>>>>> use of TSC values, so that they can correlate TSC values with linux
>>>>>>> clock_gettime() values.
>>>>>>
>>>>>> What has ART to do with correct CLOCK_MONOTONIC_RAW values?
>>>>>>
>>>>>> Nothing at all, really.
>>>>>>
>>>>>> The kernel makes use of the proper information values already.
>>>>>>
>>>>>> The TSC frequency is determined from:
>>>>>>
>>>>>>     1) CPUID(0x16) if available
>>>>>>     2) MSRs if available
>>>>>>     3) By calibration against a known clock
>>>>>>
>>>>>> If the kernel uses TSC as clocksource then the CLOCK_MONOTONIC_*
>>>>>> values
>>>>>> are
>>>>>> correct whether that machine has ART exposed to peripherals or not.
>>>>>>
>>>>>>> has tsc: 1 constant: 1
>>>>>>> 832 / 7 = 118 : 832 - 9.888914286E+04hz : OK:1
>>>>>>
>>>>>> And that voodoo math tells us what? That you found a way to correlate
>>>>>> CPUID(0xd) to the TSC frequency on that machine.
>>>>>>
>>>>>> Now I'm curious how you do that on this other machine which returns
>>>>>> for
>>>>>> cpuid(15): 1, 1, 1
>>>>>>
>>>>>> You can't because all of this is completely wrong.
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>> 	tglx
>>>>>>
>>>>>
>>>>
>>>
>>
>

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